Metallocene catalyst systems are extensively used in a variety of polymerization systems, including the polymerization of olefins. The term "metallocene" as used herein refers to a derivative of cyclopentadienyl which is a metal derivative containing at least one cyclopentadienyl-type group which is bonded to a transition metal. The transition metal is selected from Groups IVB, VB, and VIB, preferably IVB and VIB. Examples include titanium, zirconium, hafnium, chromium and vanadium. Generally, the more preferred catalysts in the polymerization of olefins are metallocenes of Zr, Hf, or Ti.
Generally, in order to obtain the highest activity from metallocene catalysts, it has been necessary to use them with an organoaluminoxane cocatalyst, such as methylaluminoxane. This resulting catalyst system is generally referred to as a homogenous catalyst system since at least part of the metallocene or the organoaluminoxane is in solution in the polymerization media. These homogenous catalysts systems have the disadvantage that when they are used under slurry polymerization conditions, they produce polymer which sticks to reactor walls during the polymerization process (generally referred to as "fouling") and/or polymer having small particle size and low bulk density which limits the commercial utility.
Various methods have been proposed in an effort to overcome the disadvantages of the homogenous metallocene catalyst systems. Typically, these procedures have involved the prepolymerization of the metallocene aluminoxane catalyst system and/or supporting the catalyst system components on a porous carrier (also known as a "particulate solid" or "support").
Another important consideration in development of metallocene catalysts is the yield of solid polymer that is employed by employing a given quantity of catalyst in a given amount of time. This is know as the "activity" of the catalyst. There is an ongoing search for metallocene catalysts and techniques for preparing such catalysts which give improved activity for the polymerization of olefins. An evaluation of these catalysts and techniques has revealed that there is still room for improvement.
An object of the present invention is to provide a new method for preparing a supported metallocene catalyst system. In accordance with another aspect of the present invention, a method is provided for polymerizing olefins using the new type of supported metallocene catalyst system.